Hey,

We know by coulombs law Force F is directly proportional to the point charges and inversely propotional to teh square of distance between them

that is,

F=K*Q1*Q2/r^2

where K is the proportionality constant.

From the above equation and question we get that

F1= Kq/r^2 and  F2 =kq/4r^2=F1/a

putting the value of F1=120 N in the above equation

F2= 120/4= 30 N which is the required answer.

I hope this helps.

All the best!

Hey!

The force between two charges varies inversely to the square of distance between the  charges. So, if the distance between the charges is doubled then the force of attraction or repulsion will become one by forth of the original force, that is it will decrease by four times.

Thank you